Extruded fiber reinforced cement matrix composites and method of making same

ABSTRACT

An extruded fiber reinforced cement matrix composite having substantially improved tensile strength with strain hardening behavior and substantially improved tensile strain, and improved flexural properties in three-point bending is made by mixing cement, water, water soluble binder, and relatively short, discontinuous reinforcing fibers, preferably short polyvinyl alcohol fibers, to provide an extrudable mixture, the extruding the mixture to shape, and curing the cement.

This is a division of Ser. No. 08/936,349, filed Sep. 24, 1997 now U.S.Pat. No. 5,891,374 which is a continuation of Ser. No. 08/490,210 filedJun. 14, 1995, abandoned, which is a continuation of Ser. No. 08/190,335filed Feb. 1, 1994, abandoned.

FIELD OF THE INVENTION

The present invention relates to an extruded discontinuousfiber-reinforced cement matrix composite and method of making same usingextrusion techniques.

BACKGROUND OF THE INVENTION

Cement and concrete are relatively brittle materials with low tensilestrength. In attempts to overcome this deficiency of cement and concretematerials, randomly oriented, short discontinuous fibers have beenintroduced into the cementitious material as a reinforcing material forthe cementitious matrix. However, addition of fibers tends to increasethe viscosity of the cementitious matrix and to render the materialdifficult to handle and place. For this reason, in bulk constructionusing conventional mixing techniques/equipment, only short fibers (e.g.25 millimeters length) and low fiber volume fractions (e.g. less than1%) have been used heretofore. For such reinforced cementitiousmaterials, the fibers do not significantly influence the tensilestrength of the matrix. Only after the matrix has cracked do the fiberscontribute to strength by bridging existing cracks.

Several techniques are used to make commercial fiber-reinforced cementproducts. The known Hatschek process was initially developed forproduction of asbestos composites and is now utilized for manufacture ofnon-asbestos, short discontinuous fiber (e.g. wood fibers and/orpolyethylene pulp) reinforced cement composites. In this process, afiber-cement mixture with excess water is deposited (e.g. roll coated)on a felt band substrate, vacuum dewatered, calendared, and cured toform a fiber reinforced cement matrix in sheet form. However, thismethod is suitable only for fiber types which retain cement particlesduring vacuum dewatering. Composites made by the Hatschek process arebrittle and only good for sheet.

U.S. Pat. No. 5,108,679 describes manufacture of discontinuousfiber-reinforced cement roofing products using the known roller andslipper process. In this process, the premixed materials including notmore than 4 weight % fibers are compressed by passage through rollersand then slipper to obtain flat reinforced sheets to which the processis limited.

Other manufacturing techniques for fiber-reinforced cement productsemploy continuous fibers rather than short, discontinuous fibers. Forexample, the known Reticem process produces cement laminate compositeswith 20 to 30 continuous fiber mesh layers. In particular, each fibermesh layer is fed from a mesh supply reel, spray coated with cement,covered with the next mesh layer that is then spray coated with cementand so on to form the multi-layered laminate that is compacted, trimmedto length, and cured.

The known pultrusion process produces continuous fiber-reinforcedstructural shapes with very high fiber volume ratios. In particular, inpractice of the pultrusion process, continuous fiber mats are fed fromstationary and roving mat creels to a cement slurry bath for coating.Then, the coated mats are formed to shape and cured under pressure.

The Reticem and pultrusion processes described above are disadvantageousin that they require continuous fibers and the processing/equipmenttechnology for incorporating the continuous aligned fibers in the cementmatrix are costly. As a result, these processes has been used for themost part in the manufacture of specialty products, such as thin sheets.

U.S. Pat. No. 4,066,723 describes production of laminated cement sheetswherein an unreinforced sheet of concrete is extruded, reinforcingfibers are then distributed onto the surface of the sheet, and thesesteps are repeated to produce a flat lamination. The process is limitedto production of flat laminated products.

It is an object of the present invention to provide a method of makingdiscontinuous fiber reinforced cement matrix composites, as well thereinforced cement composites themselves, using relatively high volumefractions of discontinuous reinforcing fibers by die extrusion toprovide improved mechanical properties in that direction.

SUMMARY OF THE INVENTION

The present invention provides a method of making a fiber reinforcedcement matrix composite having improved strength wherein hydrauliccement, water, water soluble binder for viscosity control, anddiscontinuous reinforcing fibers are mixed to provide an extrudablemixture having the reinforcing fibers substantially uniformly dispersedtherein. The mixture is extruded through an extrusion die orifice havinga desired configuration for the composite to provide increased tensileproperties of the cured composite as compared to an unreinforced cementmatrix or a cast composite of similar composition. The mixture can beextruded to preferentially align the fibers in the extrusion directionto an extent to increase tensile properties as compared to those in thetransverse direction of the extruded composite. Although aligned in thedirection of extrusion, the reinforcing fibers remain substantiallyuniformly dispersed throughout the cement matrix of the extruded shape.Alternately, the mixture can be extruded with little or no preferentialfiber alignment and yet achieve improved tensile properties in alldirections of the composite as compared to a cast composite of similarcomposition. The extruded shape then is subjected to a curing operationto cure the cement matrix. The discontinuous fiber reinforced cementmatrix can be extruded to have the configuration of a flat sheet, pipe,rod, beam, tube, honeycomb, and other structural shapes.

In practicing one embodiment of the invention, the cement preferablycomprises a hydraulic cement, such as Type I portland cement. The weightratio of water to cement preferably is within the general range of 0.2to 0.4. The discontinuous reinforcing fibers can be selected from thegroup consisting of polyvinyl alcohol, carbon, steel, polypropylene,cellulose and others and are present from about 4% to about 10% byvolume based on dry constituents of the extrudable mixture. The watersoluble binder typically is used with a water reducing agent in theextrudable mixture to adjust viscosity of the mixture to the appropriatelevel for extrusion pursuant to the invention.

In one embodiment of the invention, silica fume is mixed with the cementconstituent in formation of the extrudable mixture. The silica fumepreferably comprises silica fume powder having a size not exceedingabout 1 micron in aqueous slurry. The weight ratio of silica fume powderto cement is up to 0.30.

In a particular embodiment, the present invention provides a method ofmaking a fiber reinforced cement matrix composite having improvedtensile strength in the extrusion direction, strain hardening behaviorwith improved tensile strain (e.g. at least 1% tensile strain) andimproved flexural strength in three-point bending. In practicing thisembodiment of the invention, hydraulic cement, water, water solublebinder, and discontinuous reinforcing fibers comprising a hydrophilicpolymeric material, such as polyvinyl alcohol fibers, are mixed toprovide the extrudable mixture, the mixture is extruded to shape withthe discontinuous fibers preferentially aligned in the extrusiondirection of the extruded shape, and the cement is cured.

The present invention also provides a fiber reinforced cement matrixcomposite having a die extruded shape and improved tensile properties inall directions of the composite as compared to a cast composite ofsimilar composition.

The present invention also provides a fiber reinforced cement matrixcomposite having an extruded shape and having substantially improvedtensile strength compared to a cast composite of similar composition,strain hardening behavior with substantially improved tensile strain andimproved flexural strength in three-point bending.

The present invention provides a fiber reinforced cement matrixcomposite having an extruded shape and improved tensile properties inthe extrusion direction compared to an unreinforced matrix or a castcomposite of similar composition. The composite comprises a cured cementmatrix and discontinuous reinforcing fibers dispersed in the matrix andpreferentially aligned in the extrusion direction of the composite to anextent to increase the relative tensile properties of the curedcomposite in that direction as compared to the transverse direction byvirtue of extrusion of the cementitious mixture.

The objects, advantages and capabilities of the present invention willbecome more readily apparent with reference to the following detaileddescription of certain embodiments along with the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of an auger extruder used topractice an embodiment of the invention.

FIG. 2 is an elevational view of the shredder located in the augerextruder.

FIG. 3 illustrates graphs of tensile stress versus tensile strain forfour (4) specimens comprising unreinforced cement matrix (i.e. withoutreinforcing fibers).

FIG. 4A illustrates graphs of tensile stress versus tensile strain forthree (3) specimens comprising the cement matrix reinforced with 4volume % and FIG. 4B illustrates similar data for two (2) specimenscomprising 8 volume % discontinuous polyvinyl alcohol micro-fibers.

FIG. 5 is a graph of tensile stress versus tensile strain under cyclicloading for the cement matrix reinforced with 8 volume % discontinuouspolyvinyl alcohol micro-fibers.

FIG. 6 illustrates graphs of tensile stress versus tensile strain fortwo (2) specimens comprising the cement matrix reinforced with 4 volume% discontinuous carbon micro-fibers.

FIG. 7 illustrates graphs of tensile stress versus tensile strain fortwo (2) specimens comprising the microlite-aggregate filled cementmatrix reinforced with 4 volume % discontinuous carbon micro-fibers.

FIG. 8A illustrates graphs of tensile stress versus tensile strain forthree (3) specimens comprising the cement matrix reinforced with 4volume % and FIG. 8B illustrates similar data for two (2) specimenscomprising 8 volume % discontinuous steel micro-fibers, respectively.

FIG. 9 illustrates graphs of tensile stress versus tensile strain forthree (3) specimens comprising the cement matrix reinforced with 4volume % discontinuous polypropylene micro-fibers.

FIG. 10 illustrates graphs of tensile stress versus tensile strain fortwo (2) specimens comprising the cement matrix reinforced with 4 volume% and FIG. 11 illustrates similar data for three (3) specimenscomprising 8 volume % discontinuous cellulose micro-fibers,respectively.

FIG. 12 is a schematic elevational view of the three point bending testfixture and specimen.

FIGS. 13A, 13B are graphs of bending stress versus deflection for two(2) specimens of the cement matrix reinforced with 4 volume % and 8volume % of discontinuous polyvinyl alcohol micro-fibers, respectively.

FIG. 14 is a scanning electron micrograph (back scatter image)250×magnification of the polished cross-section (taken perpendicular toextrusion direction) of a polyvinyl alcohol fiber reinforced cementmatrix composite of the invention (8 volume % PVA fibers).

FIG. 15 is a scanning electron micrograph (back scatter image) at250×magnification of the polished surface of a carbon fiber reinforcedcement matrix composite of the invention (4 volume % carbon fibers).

FIG. 16 is a scanning electron micrograph (back scatter image) at250×magnification of the polished cross-section of a steel fiberreinforced cement matrix composite of the invention (8 volume % steelfibers).

FIG. 17 is a scanning electron micrograph (back scatter image) at250×magnification of the polished cross section of a carbon fiberreinforced cement composite of the invention (4 volume % carbon fibers).

FIG. 18 is a scanning electron micrograph at 5000×magnification of theinterface of a polyvinyl alcohol fiber and the cement matrix.

FIG. 19 is a graph of tensile stress versus tensile strain for two (2)specimens of the extruded 4 volume % carbon fiber reinforced compositesof the invention and two (2) specimens of the simple cast (i.e.non-extruded) 4 volume % carbon fiber reinforced composites.

FIG. 20 is a graph of tensile stress versus tensile strain for two (2)specimens of the extruded 4 volume % polyvinyl alcohol fiber reinforcedcomposites of the invention and three (3) specimens of simple cast (i.enon-extruded) 4 volume % carbon fiber reinforced composites.

FIG. 21 is a scanning electron micrograph at 30×magnification of asection of an extruded 4 volume % PVA fiber reinforced composite of theinvention wherein the specimen extrusion direction is the verticaldirection in the micrograph and the fibers exhibit substantial alignmentin that direction.

FIG. 22 is a scanning electron micrograph at 40×magnification of asection of an extruded 4 volume % carbon fiber reinforced composite ofthe invention wherein the specimen extrusion direction is in thevertical direction in the micrograph and the fibers exhibit little or noalignment in that direction; i.e. random fiber orientation.

FIG. 23 is a scanning electron micrograph at 30×magnification of asection of a simple cast (i.e. non-extruded) 4 volume % PVA fiberreinforced composite.

DETAILED DESCRIPTION OF THE INVENTION

Practice of the present invention involves providing an extrudablecementitious mixture comprising selected constituents suitable forproducing a fiber reinforced cement matrix composite having mechanicalproperties suited for a particular service application and extrudedlength and shape. The present invention can be practiced to produceextruded discontinuous fiber reinforced cement matrix composites havinghigh tensile and flexural strengths and toughness comparable to morecostly continuous fiber cement matrix composites with some composites ofthe invention exhibiting work hardening behavior when stressed intension. The extruded discontinuous fiber reinforced cement matrixcomposites of the invention can be extruded to a variety of shapesincluding structural shapes, such as flat sheet, contoured sheet, pipe,rod, I-beam, tube, honeycomb, and other solid or hollow shapes withoutthe need for a substrate, such as the felt band substrate of theaforementioned Hatschek process and without the need for continuousfibers of the aforementioned Reticem and pultrusion processes. Theinvention thus provides a substrateless composite reinforced withdiscontinuous fibers and extruded as a monolithic body or article ofmanufacture without laminated layers a variety of useful shapes.

The extrudable mixture typically comprises hydraulic cement, water,water soluble binder, and discontinuous reinforcing fibers inproportions to provide an extrudable cementitous dough or paste. Otheroptional constituents, such as additives and organic and/or inorganicprocessing aids and/or fillers, can be present in the uncured mixture(cementitious dough or paste) as needed to impart thixotropic rheologyand viscosity characteristics that facilitate extrusion to achievein-situ fiber alignment for a given extrusion die and mixture chemistryand density. Selection of the mixture constituents will depend on thetype, size and volume fraction of fibers present, the type of extrusionand other processing parameters to be employed, and the shape of thecomposite (i.e. the extrusion die orifice configuration) such that thefibers present in the mixture are preferentially aligned (in-situ duringextrusion) in the extrusion direction of the extruded shape, the fibersand surrounding cementitious matrix are compressed together to provideintimate interfaces therebetween and minimize porosity of the resultingfiber reinforced cement matrix composite.

The hydraulic cement included in the mixture typically comprises type Iportland cement (silicate cement) as a result of its relatively low costand ready processability, although the invention is not so limited andcan be practiced using other types of hydraulic cements, such asrapid-hardening cement, calcium aluminate cement and others as well asmixtures thereof. As is known, hydraulic cement refers to a cement thatsets and hardens in the presence of water. The hydraulic cementpreferably is used in conjunction with silica fume in the extrudablemixture. Silica fume is beneficial for reducing porosity and increasingstrength of the cured fiber reinforced cement matrix composite. Thesilica fume typically is dispersed in the extrudable mixture by additionin aqueous slurry form (50 weight % water/50 weight % silica fumepowder) as described herebelow in the Examples. The silica fume(available as force 10,000 from W.R. Grace & Co. Bedford Park, Ill.) tocement (type I portland cement) ratio was maintained at 0.18 in theExamples, although the invention is not so limited and can be practicedusing silica fume to cement ratios of 0.05 to 0.30.

Water is present in the extrudable mixture in controlled amounts toimpart, in conjunction with other additives, adequate thixotropicrheology and viscosity to the cementitous dough or paste for extrusionto shape with preferential alignment of the fibers in a load-bearingdirection of the composite. The amount of water present is controlled toa minimum in order to maintain high composite strength yet be sufficientto effect adequate cement hydration and dissolve the water solublebinders described herebelow. A typical ratio of water to hydrauliccement is 0.24 to 0.29, more generally 0.2 to 0.4, depending on theparticular constituents of the extrudable mixture. Of course, the amountof water present will depend upon the other additives used in themixture as well as the amount of aggregates, if any, used in themixture. The Examples set forth herebelow illustrate the addition ofwater to the mixture via the silica fume slurry addition, although theinvention is not limited in this regard.

Aggregate optionally can be included in the extrudable mixture toenhance rheological characteristics of the cementitious dough or paste.An illustrative aggregate comprises a lightweight aggregate availablecommercially as Microlite aggregate from Specrete-IP, Inc, Cleveland,Ohio. This lightweight aggregate is essentially a metastable amorphousaluminum silicate with a porous honeycomb microcellular structurecomposed of tiny air cells. Incorporation of such lightweight aggregatein the composite reduces it weight and improves thermal insulation ofthe composite. Generally, such lightweight aggregates are present in anamount ranging from about 10 to 50 volume % of the extrudable mixture toavoid strength reduction in the final cured composite. The invention isnot limited to the aforementioned lightweight aggregate and can bepracticed using other aggregates, such as sand, expanded polystyrenebeads, vermiculate, expanded shale and/or other materials.

A variety of discontinuous (i.e. short length). reinforcing fibers canbe used in practicing the invention. For purposes of illustration andnot limitation, the discontinuous fibers can be selected from the groupconsisting of polyvinyl alcohol (PVA) fibers, pitch based carbon fibers,steel fibers, polypropylene (PP) fibers, and cellulose fibers. Table 1here below sets forth dimensions and mechanical properties for thesefibers.

PVA fibers used in the Examples set forth herebelow were available fromKuraray Co., Ltd. Osaka, Japan, and are made from polyvinyl alcoholresin. As a result of the inherent affinity of the molecule for water(hydrophilic fiber) due to the presence of hydroxyl groups thereon andthe surface treatment (providing a tortuous surface on a microscopicscale), the PVA fibers are readily dispersed in the extrudable mixtureand provide a strong interfacial bond with the matrix in the curedcomposite. Other hydrophilic fibers that could be used in lieu of PVAfibers include, but are not limited to polyacrylic, polyethylene,polyacrylamide, or other fibers derived from vinyl acetate and anyfibers (e.g. cellulose fibers discussed below) which can promotehydrogen bonding to the backbone of the fiber molecule as a result ofbeing water soluble, water dissolvable and/or hydrolyzable by the waterpresent in the, cementitous dough.

Pitch based carbon fibers comprise a graphitic crystalline structure andexhibit a high modulus. Pitch based carbon fibers used in the Examplesherebelow were available from Kureha Chemical Industry Co., Ltd. Tokyo,Japan.

Polypropylene fibers used in the Examples comprise high molecular weightpolypropylene and are alkali resistant, have a relatively high meltingpoint, and low cost. Monofilament PP fibers were used in the Examplesherebelow and were available from W.R. Grace & Co., Bedford park, Ill.

Microsteel fibers used in the Examples herebelow were short in length(1-3 mm) and small in diameter (30-50 microns) and were easy to mix anddisperse in the extrudable mixture. Microsteel fibers used in theExamples herebelow were available from Novocon International Inc., Mt.Prospect, Ill.

Cellulose fibers used in the Examples herebelow were of the SSK typeavailable from Proctor & Gamble Co., Cincinnati, Ohio. These fiberscomprise bleached southern softwood kraft pulp liberated from slashpine. The fibers contain the naturally occurring fraction ofsummerwood-versus-springwood fibers and average fiber length.

Table 1 herebelow sets forth dimensions and mechanical properties forthe fibers used in the Examples herebelow.

TABLE 1 Tensile Fiber type Density Length Diameter Aspect ratio strengthModulus PVA 1.3 g/cc 6 mm 15 μm 400 0.9 GPa 29 GPa Carbon 1.6 g/cc 6 mm12 μm 500 0.55 GPa  45 GPa Polypropylene 0.9 g/cc 19 mm 30 μm 633 0.37GPa  3.6 GPa Steel 7.8 g/cc 1-3 mm 30-50 μm 33-60 1.4 GPa 200 GPaCellulose 1.5 g/cc 2.55 mm 30-120 μm 20-85 0.5 GPa 25-40 GPa

Certain processing aids are used in practicing the invention to achievethe proper rheology and viscosity of the extrudable mixture(cementitious dough) for extrusion to achieve in-situ fiber alignment inthe extrusion direction of the composite and other benefits describedhereabove. For example, the extrudability (workability) of thecementitious dough is modified by inclusion of one or more water solublebinders that are present to adjust the viscosity of the cementitiousdough to the desired level for extrusion. An illustrative water solublebinder for use in the invention comprises water soluble cellulose typemolecular binder comprising hydroxypropyl methylcellulose available asMETHOCEL binder from Dow Chemical Co., Midland, Mich. This bindermodifies the mixture to a medium level of viscosity. Anotherillustrative water soluble binder comprises water soluble polymer resinavailable as POLYOX binder from Union Carbide Chemicals and PlasticsCompany Inc., Danbury, Conn. This binder modifies the mixture to arelatively low level of viscosity. Although the Examples herebelowemploy a 65 weight % METHOCEL/35 weight % POLYOX proportion bindersystem, to provide desired viscosity, the invention can be practicedusing a single water soluble binder, such as either the METHOCEL binderor the POLYOX binder discussed above. The invention is not limited tothese particular binders, however. The ratio of the aforementionedbinder(s) to cement was maintained about 0.01 in the Examples herebelowto reduce moisture sensitivity of the cured composite.

The extrudability (workability) of the cementitious dough also ismodified by inclusion of a water reducing agent. An illustrative waterreducing agent comprises high range water reducer (HRWR) available fromW.R. Grace & Co., Bedford Park, Ill. and comprising linear polymercontaining a sulfonic acid group attached to the polymer backbone atregular intervals. The sulfonic acid groups neutralize surface chargeson the cement particles, thereby facilitating dispersion in thecementitious paste. Once the water content of the cementitous paste isdetermined, the quantity of HRWR was varied in the Examples herebelow tomodify workability of the dough to the degree for extrusion pursuant tothe invention. The weight ratio of the water reducing agent, HRWR, tocement in the extrudable mixture generally is controlled at up to 0.06.

The fibers are present in the extrudable mixture in a volume fraction toachieve improved tensile properties (i.e. tensile strength and tensilestrain) in the final cured composite suited to the intended serviceapplication and yet maintain an extrudable mixture as describedhereabove. Typically, the fibers are present generally from about 4 toabout 10 volume fraction based on the aforementioned mixtureconstituents in dry form.

In the Examples herebelow, the fibers were present in 4 volume % and 8volume % of the dry mixture constituents for purposes of illustration,not limitation. All volume fractions of fibers set forth herein arebased on the dry extrudable mixture constituents.

In extruding the cementitious dough pursuant the invention, theextrusion die orifice forms the dough under compressive and shear forceseffective to configure the dough to the die shape and, if desired, toachieve in-situ preferential alignment of the fibers in the extrusiondirection of the extruded shape (see FIG. 21), compression of the fibersand surrounding cementitious matrix together to provide intimateinterfaces therebetween and reduced porosity of the resulting fiberreinforced cement matrix composite shape. The extrudable mixture(cementitious dough) can be extruded to shape under conditions of highshear and high compressive forces to achieve in-situ preferentialalignment of the fibers in the extrusion direction of the extruded shapeto an extent to increase the relative tensile properties of the curedcomposite in that direction as compared to the transverse direction.

In practicing a preferred embodiment of the invention, the cementitiousdough is extruded using a auger extruder (screw type extruder) having asuitable die orifice for the intended extruded shape. An illustrativeauger extruder capable of extruding 2000 pounds of cementitious doughper hour is shown schematically in FIG. 1 and is available as StarkeyModel 990H-1, 3HP motor from Starkey Machinery Inc., Galion, Ohio. Thisauger extruder was used in the Examples set forth herebelow to extrudethe cementitious dough to desired shape.

The invention is not limited to the auger extruder illustrated in FIG. 1and can be practiced using other die extruder devices, such as a ramextruder, wherein a extruder ram is powered by a hydraulic press to ramthe cementitious dough in an extruder barrel through a suitable extruderdie orifice for the intended extruded shape. Ram extrusion was used inpracticing the invention to determine proper proportions of theconstituents of the cementitious dough.

The extent to which the reinforcing fibers are aligned in-situ in theextrusion direction, and thus the mechanical properties in thatdirection, can be controlled by adjustment of the extrusion pressureexerted on the cementitious dough. This pressure can be varied by, forexample, viscosity of the cementitious dough, rate of extrusion, and diedesign.

The reinforcing fibers can be aligned in the extrusion direction tovarying extents needed to provide improved mechanical properties, suchas tensile strength, in that direction. FIG. 21 illustrates a highdegree of preferential alignment of PVA reinforcing fibers to optimizetensile strength in the extrusion direction; however, lesser degrees ofpreferential fiber alignment in the extrusion direction can be provided.within the scope of the invention as required. for other serviceapplications where less relative strength is required in the extrusiondirection.

Thus, the invention can be practiced over a range of parameters wherein,on one hand, little or no preferential alignment of the reinforcingfibers is achieved in the extrusion direction as illustrated, forexample, in FIG. 22 and wherein, on the other hand, substantialpreferential fiber alignment is achieved in the extrusion direction asillustrated, for example, in FIG. 21. However, even when the extrusionand other parameters are selected to provide little or no preferentialfiber alignment in the extrusion direction, the resulting extruded,cured fiber reinforced cement matrix composite exhibits increasedtensile properties in all directions (isotropic properties) of thecomposite as compared to a cast composite of similar composition; i.e.having similar fibers/fiber fraction in a similar cement matrix.

When substantial preferential fiber alignment is achieved in theextrusion direction by appropriate selection of extrusion and otherparameters, the extruded and cured composite exhibits dramaticallyimproved tensile properties in the extrusion direction as compared to adirection transverse to the extrusion direction for demanding and severeservice applications that may be encountered. Generally, the tensileproperties of the composite can be tailored in this manner to suit avariety of service application requiring improved tensile properties ofa more isotropic nature or of an anisotropic nature (i.e. in thedirection of extrusion of the composite). Moreover, the tensileproperties in directions other than the extrusion direction (e.g.transverse direction) are still increased in practicing the invention ascompared to a cast composite of similar composition. For example,tensile properties of the composite of the invention with little or nofiber alignment are compared in all directions over a similar castcomposite. The composite having preferentially aligned fibers exhibitsincreased tensile properties not only in the extrusion direction butalso other directions as compared to a cast composite.

Dies of a variety of geometries and shapes can be used in practicing theinvention. For purposes of illustration and not limitation, extrusiondies having a 0.25 inch diameter orifice and 1.0 inch diameter orificewere used to produce extruded lengths of respective 0.25 and 1.0 inchdiameter solid uncured cementitious rods. A die orifice having a widthof 3 inches and height of 0.25 inch was used to produce extruded lengthsof rectangular cross-section uncured cementitious flat sheet. A 1 inchby 1 inch honeycomb die orifice was used to produce extruded lengths ofuncured cementitious honeycomb. The extrusion dies are made of stainlesssteel to withstand the forces and abrasion associated with extrusion ofthe cementitious doughs or pastes.

The aforementioned mixture components are premixed to proper viscosityprior to introduction to the extruder. A typical premix schedule used inpractice of the Examples herebelow involved preparing a dry premixtureof type I portland cement particles and water soluble binder particlesby placing the particles in a cylindrical container and shaking androlling the container until the binder particles were fully dispersed inthe cement. Alternately, the cement particles and binder particles weredry mixed in a conventional high frequency vibration mixer available asOmni mixer from Chiyoda Technical & Industrial Co., Ltd., Tokyo, Japan.

A suitable silica fume slurry was prepared and then mixed with theparticular discontinuous reinforcing fibers to be incorporated in thecement matrix to insure all fibers were wetted. The silica fumeslurry/fibers were mixed in a conventional high shear mixer; e.g. typeG60 read vertical mixer available from G.S. Blakelee & Co., Chicago,Ill. at a nominal mixer speed of 450 rpm.

A cement/binder premixture then was slowly added to the slurry/fiberswhile a selected amount of water also was added slowly and continuously.The batch began in a short time (e.g. 10) minutes to gain cohesion amongthe particles present due to the reaction of the water soluble binderwith the water present. However, the required viscosity for extrusionwas not achieved at this point. High range water reducer was then addedto replace the remaining amount of water required to attain a desiredlevel of viscosity for extrusion. The water reducer was added slowly inliquid form into the batch until a dough-like viscosity was obtained.The speed of the mixer was increased from nominal 250 rpm to nominal 450rpm so that the dough-like mixture (cementitious dough) was quicklykneaded several times to reach a uniform dough mix of putty-likeconsistency. The premixing operation to provide the dough cementitiouspaste or mixture took about 5 to 10 minutes.

The extrudable mixture (cementitious dough) then can be suppliedimmediately (e.g. within 10 minutes) to the extruder for extrusion todesired length and shape through the die orifice 10. In. practicing theinvention using the extruder shown in FIG. 1, the cementitious dough issupplied to the pug mill chamber of the extruder. In the pug millchamber, the components of the cementitous dough are further mixed andpushed through a shredder S shown in FIG. 2. The geometry of theshredder S is chosen as needed to accommodate different fiber geometry(e.g. longer length) to avoid shredding of the fibers by the shredderblades. In particular, the space or distance between the shredder bladesis chosen to this end.

In conducting the Examples herebelow, the shredder blades are angled atan angle of 45° relative to the axis of the extruder and are spacedcircumferentially apart by 0.5 inches (blade edge-to-blade edge).

At the position of the shredders, a vacuum system communicates to thepug mill chamber to remove air from the cementitious dough. Thus, whilethe cementitious dough passes through the shredder S, it isdeflocculated by the shredder orifices and also deaerated by the vacuumsystem. A vacuum level of 25-30 inches of mercury was used to this end.

After leaving the shredder, the cementitous dough was coagulated againand pushed through the extrusion die orifice 10 by the extrusion augershown in FIG. 1.

The cementitious dough is extruded to a shape corresponding to the dieorifice 10 and to an extruded length determined by the compositedimensions selected. As shown in FIG. 1, the extrusion die includes atapered, converging barrel B in the end of which is an orifice 10.

For purposes of illustration, the auger extruder described hereabove andused in the Examples herebelow extruded cementitious dough at about ½foot in length per second with a rectangular cross of 3 inches width by0.25 inch in thickness.

The extruded cementitious dough composite shape typically is cut toselected length and then covered for about 24 hours with one or moremoisture impermeable plastic sheets to achieve initial cure of thecement matrix in an isolated moist environment. After about 24 hours,the initially cured extruded cementitious dough composite shape issubmerged in a water tank at room temperature (20° C.) for curing of thecement matrix. Curing was continued for 28 days in the water tank.However, other curing times and environments can also be used inpracticing the invention.

For purposes of illustration, extruded cementitious dough sheets(specimen composition given in Table 2) that were 3 wide inches by 0.25inch thick by 12 inches length were sandwiched between flat sheets of 24inch×24 inch×1½ inch thickness plexiglass. About 5 psi pressure wasapplied to the upper sheet by a weight to retain flatness. Thesandwiched extruded sheets were cured in a moist room (100% relativehumidity) for 24 hours under such pressure.

Table 2 herebelow sets forth various Examples of extrudable mixtures(cementitious dough) that were prepared, extruded and cured in themanner described hereabove. A control Example comprised only the cementmatrix made from the components listed in the Table without anyreinforcing fibers present.

TABLE 2 Fiber fraction Specimen Fiber type W/C SF/C Binder/C HRWR/CAggregate 0% control none 0.27  0.18 0.009  0.43% 0 4% pva4 PVA 0.2860.18 0.009 4.3% 0 c4 carbon 0.256 0.18 0.009 2.8% 0 st4 microsteel 0.2560.18 0.01  3.0% 0 cell4 cellulose 0.287 0.18 0.009 4.3% 0 pp4polypropylene 0.268 0.18 0.009 4.6% 0 cm4 carbon 0.243 0.18 0.006 5.6%microlite 10% C 8% pva8 PVA 0.282 0.18 0.009 5.6% 0 st8 microsteel 0.2560.18  0.0086 3.0% 0 cell8 cellulose 0.288 0.18  0.0082 6.0% 0 C: Type IPortland cement; W: water; SF: silica fume in slurry; HRWR: high rangewater reducer; Binder: 65% METHOCEL + 35% POLYOX

As mentioned hereabove, various extruded composite shapes were produced.For example, 1 inch by 1 inch square cross section honeycomb compositesof 5 inches length were produced from the cementitious compositiondesignated specimen “control” in Table 2. One inch diameter solid rodsof 6 inches length were produced from the cementitious compositiondesignated specimen “c4” in Table 2. Solid rods having a diameter of0.25 inch were produced from the cementitious composition designatedspecimen “c4” in Table 2. Flat sheets of 3 inches width and 0.25thickness and 12 inches length were produced from the cementitiousspecimens in Table 2.

Extruded composite mechanical property test specimens were produced asflat sheets having a rectangular cross section with a width of 3 inchesand thickness of 0.25 inch and a length of 12 inches. The specimens wereproduced from the Example compositions set forth in Table 2 in themanner described hereabove. All specimens were initially cured for 24hours under plastic sheeting and then completely cured in the water tankfor 28 days after extrusion.

Specimens for direct tension testing were made by cutting the extrudedspecimens into two 1 inch wide test specimens to provide final testspecimen dimensions of 1 inch width, 0.25 inch thickness and 12 inchesin length. Prior to tension testing, the opposite ends of the specimenswere reinforced by gluing pieces of steel sheet to the ends to assurethat failure of the specimen occurred within the 3 inch long gage lengthprovided.

Direct tension testing was conducted on an MTS testing machine whereintensile strain was measured by 2 linear variable displacementtransducers (LVDT) with the gage length of 3 inches. This displacementwas used as a feedback control. Statistical methods were used todetermine the strength and the Young's modulus of the composites byaveraging the test results from at least 5 test specimens.

FIG. 3 illustrates the tensile stress versus tensile strain forunreinforced cured cement matrix specimens made from the “control”Example composition set forth in Table 2. The unreinforced matrixspecimens exhibited typical brittle behavior, breaking suddenly at peaktensile stress. The average tensile strength of these specimens was487.5 psi and the average Young's modulus was 4.01×106 psi. These valuesrepresent control or reference values for comparison to the strength andmodulus values determined for the extruded fiber reinforced cementmatrix composite specimens of the invention.

FIGS. 4A and 4B illustrate the tensile stress versus tensile strain forPVA fiber reinforced cured cement matrix composite specimens made fromthe Example compositions listed for specimens “pva4” and “pva8” in Table2. The 4% and 8% PVA fiber reinforced composite specimens exhibitedstrain hardening behavior in the tension tests. The descending andascending points along the curves after the bend-over-point was reachedindicate that the composite specimens were still capable of beingreloaded even after the cement matrix suffered multiple cracking. Acomparison of FIGS. 4A and 4B indicates that load transfer from matrixto fibers and from fibers to matrix was stabilized by the higher volumefraction of fibers of specimen “pva8”. The 4% and 8% PVA compositespecimens exhibited an average tensile strength of about 1000 psi andstrain hardening behavior with a strain of at least about 1%. Theaverage Young's modulus of the specimens is shown in Table 3.

FIG. 5 illustrates the tensile stress versus tensile strain under cyclicloading for PVA fiber reinforced cured cement matrix composite specimensmade from the Example compositions listed for specimens “pva8” in Table2. It is evident that, after several load and unload cycles, the strainhardening response of the composite remains and no stiffness degradationwas observed.

FIG. 6 illustrates the tensile stress versus tensile strain under cyclicloading for carbon fiber reinforced cured cement matrix compositespecimens made from the Example composition listed for specimens “c4” inTable 2. Both strength and toughness of the composite were enhancedcompared to the unreinforced cement matrix, FIG. 3. No strain hardeningbehavior was observed for specimen “c4”.

FIG. 7 illustrates the tensile stress versus tensile strain under cyclicloading for carbon fiber reinforced cured cement matrix compositespecimens made from the Example composition listed for specimens “cm4”in Table 2 wherein lightweight aggregate is added to the composition.Although the stress-strain curves of the aggregate-filled composite werecomparable to those of the unfilled specimen “c4”, the strength andtoughness of the aggregate-filled composite specimen “cm4” were lower.This reduction in strength and toughness might be attributable to porouscellular structure of the lightweight aggregate. This suggests usingsolid aggregate filler in lieu of the porous filler used in the Example.

FIGS. 8A and 8B illustrate the tensile stress versus tensile strain formicrosteel fiber reinforced cured cement matrix composite specimens madefrom the Example compositions listed for specimens “st4” and “st8” inTable 2. The 8% microsteel composite specimen exhibited a tensilestrength as high as 1800 psi compared to a tensile strength of 820 psifor the 4% microsteel composite specimen. The toughness of the 4% and 8%microsteel composite specimens was about the same, however. The averageYoung's modulus of the specimens is shown in Table 3. The 4% and 8%microsteel fiber reinforced composite specimens did not exhibit strainhardening behavior in the tension tests. With a longer steel microfiber,strain hardening may be possible.

FIG. 9 illustrates the tensile stress versus tensile strain for thepolypropylene (PP) fiber reinforced cured cement matrix compositespecimens made from the Example composition listed for specimens “pp4”in Table 2. The specimens exhibited strain hardening behavior in thetension tests. The tensile strength of the composite is very lowcompared to the unreinforced cement matrix, FIG. 3. The low tensilestrength is perhaps attributable to lower packing density due to longerfiber length.

FIGS. 10 and 11 illustrate the tensile stress versus tensile strain forcellulose fiber reinforced cured cement matrix composite specimens madefrom the Example compositions listed for specimens “cell4” and “cell8”in Table 2. The 4% cellulose composite specimens was only slightlyimproved compared to the unreinforced cement matrix, FIG. 3. On theother hand, the strength and toughness of the 8% cellulose compositespecimens were substantially improved compared to the unreinforcedcement matrix. A high volume fraction of cellulose greater than 4% thusis necessary to achieve substantially improvements in tensile strengthand toughness of the composite. The average Young's modulus of thespecimens is shown in Table 3. The 4% and 8% microsteel fiber reinforcedcomposite specimens did not exhibit strain hardening behavior in thetension tests.

Table 3 herebelow summarizes the mechanical properties of the abovediscussed test composite specimens of the invention as well as theunreinforced cement matrix control specimen listed in Table 2.

TABLE 3 Fiber fraction Specimen Fiber type Tensile strength Young'smodulus 0% control none 488 ± 20 psi 4.00 ± 0.7 × 10⁶ psi 4% pva4 PVA783 ± 40 psi 3.47 ± 0.5 × 10⁶ psi c4 carbon 775 ± 75 psi 2.41 ± 0.8 ×10⁶ psi st4 microsteel 819 ± 85 psi 3.64 ± 0.5 × 10⁶ psi cell4 cellulose525 ± 25 psi 1.94 ± 0.9 × 10⁶ psi pp4 poly- 338 ± 42 psi 2.67 ± 0.6 ×10⁶ psi propylene cm4 carbon 638 ± 90 psi 3.42 ± 0.4 × 10⁶ psi 8% pva8PVA 985 ± 80 psi 2.31 ± 0.6 × 10⁶ psi st8 microsteel 1390 ± 20 psi  3.87± 0.3 × 10⁶ psi cell8 cellulose 807 ± 13 psi 2.36 ± 0.2 × 10⁶ psi

A conventional three point bending test apparatus and specimen, FIG. 12,were employed to determine the flexural performance of the 4% and 8% PVAcomposite specimens “pva4” and “pva8” in Table 2. specimen length was 12inches, width was 1 inch and thickness was 0.196 inch. The length of thespan of the specimen was 7.5 inches. The radius of the lower supportsand upper plunger was 0.75 inch. FIGS. 13A and 13B comprise bendingstress versus deflection curves for the “pva4” and “pva8” specimens,respectively.

FIGS. 13A and 13B indicate a large visible deflection at the peakbending load. The maximum deflection of the 4% PVA fiber compositespecimens “pva4” was 0.5 inches in a span length of 7.5 inches with aflexural strength of 2700 psi. The maximum deflection of the 8% PVAfiber composite specimens “pva8” was 0.8 inches in a span length of 7.5inches with a flexural strength of 4000 psi. Thus, the 4% and 8% PVAreinforced cement matrix composite specimens provide flexible fiberreinforced cementitious composites which are comparable to cementitiousmaterials reinforced with continuous fibers.

FIGS. 14, 15, and 16 are scanning electron micrographs (back scatterimage) at 250× magnification of the polished cross-section of a PVAfiber reinforced cement matrix composite of the invention (8 volume %PVA fibers), the polished cross-section of a carbon fiber reinforcedcement matrix composite of the invention (4 volume % carbon fibers), andthe polished cross-section of a steel fiber reinforced cement matrixcomposite of the invention (8 volume % steel fibers), respectively.

The fiber distribution or dispersal in the cement matrix appears to beuniform for all of these composites. The fibers in FIGS. 14 and 15appear as black constituents. From FIG. 16, the microsteel fibers do nothave a well defined size/shape as various size/shapes are evident. Themicrosteel fibers resemble the grey hydration products constituent andthus are difficult to identify. Composites containing PVA and microsteelfibers exhibit few shrinkage cracks. This is attributed to the purecement and silica matrix used in the dough compositions. This can beovercome by modifying the matrix by incorporating fine sand and/orshrinkage resistant admixtures to the matrix.

The carbon composites exhibit major defects and also more shrinkagecracks as shown in FIG. 15 and FIG. 17, which is a scanning electronmicrograph (back scatter image) at 250×magnification of the polishedcross section of a carbon fiber reinforced cement composite of theinvention (4 volume % carbon fibers). Study of these defects indicates apossible association with bunches of undispersed fibers which indicatefiber balling during mixing of the cementitious dough. Modifications tothe mixing procedure used in the Examples can be made to improvedispersal of carbon fibers in the cementitious dough and thusfiber/cement matrix bonding in the composite, resulting in improvedmechanical properties.

FIG. 18 is a scanning electron micrograph at 5000×magnification of theinterface of a PVA fiber and the cured cement matrix for PVA fiberreinforced composite (4 volume % PVA fibers). The interface evidencesvery good bonding between the PVA fiber and the cement matrix. Such goodbonding results in improved strength and toughness as well as strainhardening behavior. When longer steel microfibers are used, strainhardening may be possible.

The specimens of Table 3 all exhibited preferential fiber alignment inthe extrusion direction to an extent similar to FIG. 21.

The extruded fiber reinforced cement matrix composites of the inventionwere compared to cast (non-extruded) composites comprising similarfibers/cement matrices. Comparison specimens included 4 volume % PVAfiber and 4 volume % carbon fiber in cementitous compositions set forthin Table 4 herebelow.

TABLE 4 Tensile fiber fiber strength Modulus fraction specimen processtype W/C SF/C Binder/C HRWR/C (psi) (psi) 4% c4 extruded carbon 0.2480.18 0.014   2% 966 2.6 × 10⁶ cc4 cast carbon 0.40  0.18 0   2% 390 0.8× 10⁶ 4% pva4 extruded PVA 0.286 0.18 0.009 4.3% 830 3.5 × 10⁶ cpva4cast PVA 0.286 0.18 0 6.3% 572 1.1 × 10⁶ C: Type I Portland cement; W:water; SF: silica fume in slurry; HRWR: high range water reducer;Binder: 65% METHOCEL + 35% POLYOX.

The extruded fiber reinforced cement matrix composite specimens weremade in the manner described hereabove. The cast specimens were made bycasting the cementitous dough or paste in molds measuring 1 inch wide by0.5 inch thick by 10 inches long. In making the cast specimens, thepremixture of cement and silica fume slurry was mixed with a premixtureof water and high range water reducer (HRWR) in proportions pursuant toTable 4. The appropriate fibers were then slowly fed into the resultingmixture. In making the cast carbon fiber composite specimens “cc4” ofTable 4, more water was used while maintaining the HRWR at a constantamount. In making the cast PVA fiber composite specimens “cpva4” ofTable 4, more HRWR was used while maintaining the water at a constantamount. These adjustments were made to improve castability of themixtures (cementitious dough). Each mixture was immediately cast into anappropriate mold. The mixture was cured in the mold for 24 hours andthen demolded. This specimen was then immersed in water for furthercuring for 28 days.

FIG. 19 compares the tensile stress versus tensile strain curvesobtained for 4% carbon fiber composite specimens “c4” and “cc4” of Table4 using the test procedure described hereabove. The tensile strength ofthe extruded composite specimens “c4” was twice as high as that of thecast specimens “cc4”. The energy density (the area under the tensilestress-tensile strain curve) released by the extruded composite specimen“c4” was much larger than that released by the cast specimen “cc4”″,indicating a large increase in the toughness of the composite made byextrusion.

FIG. 20 compares the tensile stress versus tensile strain curvesobtained for 4% PVA fiber composite specimens “pva4” and “cpva4” ofTable 4 using the test procedure described hereabove. The tensilestrength and the toughness of the extruded composite specimens “pva4”was much higher than those of the cast specimens “cpva4”. Moreover, theextruded PVA composite specimens exhibited strain hardening behavior andmultiple cracking in the tension tests, although multiple crackingoccurred occasionally in the cast specimens “cpva4”.

The microstructures of substantially preferentially aligned extrudedfiber composite specimen and cast composite specimens were quitedifferent. In particular, in FIG. 21, the extruded PVA fiber compositeexhibited significant preferential alignment of the fibers along thedirection of extrusion (corresponds to the vertical direction in FIG.21) even though the fibers are substantially uniformly distributedthroughout the cured cement matrix. For example, a majority of thefibers in FIG. 21 are aligned in the extrusion direction. The fiberswill have the maximum contribution to the load carrying capacity of thecomposite when they are preferentially aligned in the load-bearingdirection of the composite in a service application. The fibers alsowill achieve maximum packing density when they lie in the samedirection. In contrast, in FIG. 23, the cast PVA fiber compositeexhibited random orientation of the fibers throughout the cured cementmatrix.

As mentioned hereabove, the invention also can be practiced usingparameters where there is little or no preferential fiber alignment inthe extrusion direction and yet still achieve increased tensileproperties are achieved in all directions of the composite as comparedto cast composites of similar composition. FIG. 22 illustrates anextruded composite microstructure of the invention to this end; i.e.where there is little or no preferential fiber alignment in theextrusion direction. The composite specimen comprised 4 volume % carbonfiber in a cement matrix (specimen “c4” in Table 2). This specimen wasextruded using the auger extruder described hereabove but whereextrusion parameters were varied (e.g. by decreasing extruder speed andincreasing dough viscosity) to achieve little or no fiber alignment inthe extrusion direction in the cement matrix. The carbon fiber clumpingvisible in FIG. 22 is discussed above.

The extruded fiber reinforced cement matrix composites of the inventioncan used in a variety of service applications. For example, commercialand residential uses would involve flat and corrugated sheet roofingelements, exterior and interior wall panels, equipment screens, fasia,facades and soffits, substrates for tiles, window sills and stools,stair treads and risers, substrates for coatings, utility buildingcladding panels and other myriad applications. Agricultural uses wouldinvolve farm buildings, sidings, stalls and walls, poultry houses andincubators, green house panels and work surfaces, fencing, sun screensand others. The extruded fiber reinforced composites of the inventioncan be made not only as flat sheets or panels but also as shapedconfigurations including, but not limited to, I-beams, channels, pipes,honeycomb, tubes, and other hollow shapes.

Although the invention has been described hereabove with respect tocertain embodiments thereof, the invention can be subject tomodifications, changes, and adaptations to be considered within thescope of the invention as set forth by the appended claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. An extruded fiberreinforced cement matrix composite having a die extruded shape andimproved tensile properties in one or more directions compared to a castcomposite of similar composition, comprising a cured cement matrix anddiscontinuous polyvinyl alcohol reinforcing fibers dispersed in thematrix.
 2. An extruded fiber reinforced cement matrix composite having adie extruded shape and improved tensile properties in all directionscompared to a cast composite of similar composition, comprising a curedcement matrix and discontinuous polyvinyl alcohol reinforcing fibersdispersed in the matrix and preferentially aligned in an extrusiondirection of the composite.
 3. An extruded fiber reinforced cementmatrix composite having an extruded shape and having a cured cementmatrix with discontinuous polyvinyl alochol reinforcing fibers dispersedin the matrix to provide a tensile strength of at least 750 psi in anextrusion direction, strain hardening behavior with at least 1% tensilestrain, and improved flexural strength in three-point bending comparedto unreinforced matrix.
 4. The composite of one of claim 2 or 3 whereinthe fibers are present from 4% to 10% by volume based on dryconstituents of the composite.
 5. An extruded fiber reinforced cementmatrix composite having an extruded shape and having improved tensileproperties in all directions compared to unreinforced matrix, strainhardening behavior with tensile strain, and improved flexural strengthin three-point bending compared to unreinforced matrix, comprising acured cement matrix and discontinuous polyvinyl alcohol reinforcingfibers dispersed in the matrix and preferentially aligned in anextrusion direction to an extent to provide increased tensile propertiesin said extrusion direction, provide said strain hardening behavior andprovide said flexural strength.